1. Field of the Invention
The present invention relates generally to an optical scanning apparatus and an image forming apparatus using the same. More particularly, the present invention relates to an optical scanning apparatus and an image forming apparatus using the same that are suitably applicable to apparatuses such as, for example, laser beam printers or digital copiers employing an electrophotographic process in which image information is formed using signals from a synchronizing-signal detecting means for controlling writing start in the main scanning direction when a beam emitted from a light source means is deflected by a deflection means and then passes through an fxe2x8ax96 lens in a scanning optical system to optically scan a surface to be scanned and thus to form the image information.
2. Related Background Art
In conventional optical scanning apparatuses such as laser beam printers, a light beam emitted from a light source means is optically modulated according to an image signal. Then the optically modulated light beam is deflected periodically by a deflection means composed of, for instance, a polygon mirror and then is focused in a spot shape on a surface of a photosensitive recording medium by a scanning optical system having the fxe2x8ax96 characteristics to optically scan the recording medium surface. Thus, the image information is recorded.
FIG. 6 is a perspective view showing main part of a conventional optical scanning apparatus. In FIG. 6, a divergent light beam emitted from a light source means 1 in a laser unit 3 is converted into a substantially parallel beam by a collimator lens in the unit 3. The substantially parallel beam enters a cylindrical lens 4 with its width limited by a stop in the unit 3. Among the parallel beam that has entered the cylindrical lens 4, the beam in a plane in the main scanning direction emerges in the unchanged state and that in a plane in the sub-scanning direction is converged, which then form a substantially linear image on a deflecting/reflecting surface 5a of a deflection means 5 composed of a polygon mirror.
The light beam (indicated with a broken line) reflected and deflected by the deflecting/reflecting surface 5a of the polygon mirror 5 is led onto a recording medium surface (the surface to be scanned) 10 through a scanning optical system 7 having the fxe2x8ax96 characteristics including two fxe2x8ax96 lenses 7a and 7b. The polygon mirror 5 is rotated by a driving means 6 at a substantially constant angular velocity, so that the recording medium surface 10 is optically scanned at a substantially constant velocity. Thus, a latent image is formed using potential difference.
Part (indicated with a solid line) of the light beam reflected and deflected by the deflecting/reflecting surface 5a of the polygon mirror 5 passes through the scanning optical system 7 and then its traveling direction is changed by a reflecting mirror (BD mirror ) 11 used for synchronization detection. Afterward, the part of the beam is converged on a synchronization detecting means (BD sensor) 13 by a condenser lens (BD lens) 12 used for synchronization detection. Thus, a horizontal synchronizing signal is formed.
In a recent optical scanning apparatus, its scanning optical system having the fxe2x8ax96 characteristics has been made compact in size with the increase in demand for making the optical scanning apparatus compact in size. Methods of making the scanning optical system compact in size include, for example, increasing the angle of view of the scanning optical system and disposing the fxe2x8ax96 lens near the polygon mirror.
Methods of reducing the width in the main scanning direction of the fxe2x8ax96 lens include arranging an optical path for a synchronizing light beam (BD light beam) that is different from one for a scanning beam.
The scanning beam may not be focused at a desired position on the surface to be scanned and thus the image quality may be degraded unless the fxe2x8ax96 lens is disposed in a predetermined position with high precision. When optical elements are gathered in the vicinity of the polygon mirror to reduce the size of the whole apparatus, the synchronizing light beam and the fxe2x8ax96 lens physically interfere with each other and thus the synchronizing light beam may be blocked.
On the other hand, Japanese Patent Application Laid-open No. 11-311749 discloses an optical scanning apparatus in which an inclined part for securing an optical path of a synchronizing light beam is provided at an end of an fxe2x8ax96 scanning optical system as a scanning-beam imaging lens and thus the synchronizing light beam passes by the inclined part to enter a synchronization detecting means.
In Japanese Patent Application Laid-open No. 11-311749, however, due to the formation of the inclined part, the thickness of the imaging/scanning lens is reduced in the inclined part and this causes a problem of stiffness deterioration. In addition, it is difficult to provide a positioning reference in the inclined part. The positioning reference should therefore be provided in a place closer to the optical axis of the imaging/scanning lens relative to the inclined part. As a result, the spacing between the positioning references arranged in the longitudinal direction of the imaging/scanning lens is reduced. This causes the increase in rotational deviation of the imaging/scanning lens.
Furthermore, in Japanese Patent Application Laid-open No. 11-223789, in FIG. 1, a laser beam emitted from a laser unit 5 passes through a laser beam transmission hole 11a provided in a fxe2x8ax96 lens 11, enters a rotary polygon mirror 4, and then optically scans a photosensitive member surface 9 through the fxe2x8ax96 lens 11.
The rotary polygon mirror 4 allows a laser beam that has passed through an inner portion of the fxe2x8ax96 lens 11 among the laser beam reflected by the rotary polygon mirror 4 to enter a light-receiving element 8 for detecting a synchronizing signal. Thus, a horizontal periodic signal is obtained from the light-receiving element 8.
The present invention is intended to further improve the optical scanning apparatuses disclosed in the publications described above. It is an object of the present invention to provide an optical scanning apparatus in which a wide-angle scanning optical system can be used, the scanning optical system can be disposed closely to a deflection means, and the reduction in size of the whole optical scanning apparatus can be achieved easily. It is also an object of the present invention to provide an image forming apparatus using the optical scanning apparatus.
The present invention is intended to solve the problems of stiffness deterioration and increased rotational deviation of the scanning/imaging lens disclosed in Japanese Patent Application Laid-open No. 11-311749.
According to a first aspect of the present invention, there is provided an optical scanning apparatus, characterized in that: the optical scanning apparatus includes: a light source means: a deflection means with a deflecting/reflecting surface, the deflection means deflecting an incident light beam from the light source means by the deflecting/reflecting surface; a scanning optical system including at least one scanning optical element for leading the light beam deflected by the deflection means onto a surface to be scanned and forming an image as a spot on the surface to be scanned; and a synchronization detecting means for obtaining a scanning start position signal with respect to a main scanning direction on the surface to be scanned; and in that the scanning optical element is provided with a light-beam passage for allowing a light beam traveling toward the synchronization detecting means to pass through, the light-beam passage being provided in an inner portion of the scanning optical element located outside an effective portion of the scanning optical element.
According to a second aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that the scanning optical element is provided with a positioning reference at an end in a longitudinal direction of the scanning optical element, and the positioning reference is located within an area where the light-beam passage is present or on an outer side of the area with respect to an optical axis.
According to a third aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that the scanning optical element is provided with a positioning reference at each of one end having the light-beam passage and the other end in a longitudinal direction of the scanning optical element, and the positioning reference at the one end having the light-beam passage is located within an area where the light-beam passage is present or on an outer side of the area with respect to an optical axis, in the longitudinal direction of the scanning optical element.
According to a fourth aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that the scanning optical element having the light-beam passage is disposed in a place closest to the deflection means.
According to a fifth aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that an angle between a principal ray of part of the light beam deflected by the deflection means traveling toward an end within a scanning effective width on the surface to be scanned and a principal ray of part of the light beam deflected by the deflection means traveling toward the synchronization detecting means is set not to exceed 3xc2x0.
According to a sixth aspect of the present invention, in the second aspect of the invention, the optical scanning apparatus is characterized in that the positioning reference is provided on a surface of the scanning optical element at the deflection means side so as to position the scanning optical element with respect to a direction of the optical axis.
According to a seventh aspect of the present invention, in the second aspect of the invention, the optical scanning apparatus is characterized in that the positioning reference is provided on a surface substantially parallel to a cross section in the main scanning direction of the scanning optical element so as to position the scanning optical element with respect to a sub-scanning direction.
According to an eighth aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that the incident light beam traveling toward the deflection means is incident on the deflection means from a direction oblique to an optical axis in a main scanning plane, the light-beam passage for allowing a light beam traveling toward the synchronization detecting means to pass through is provided on a side opposite to the light source means in the main scanning direction with respect to the optical axis.
According to a ninth aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that the light-beam passage for allowing a light beam traveling toward the synchronization detecting means to pass through is provided at one end in a longitudinal direction of the scanning optical element, and a light-beam passage for allowing a light beam traveling toward the deflection means to pass through is provided at the other end in the longitudinal direction of the scanning optical element.
According to a tenth aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that the scanning optical element is a scanning lens.
According to an eleventh aspect of the present invention, in the first aspect of the invention, the optical scanning apparatus is characterized in that the light-beam passage is a notch portion provided by cutting a part of an end of the scanning optical element.
According to a twelfth aspect of the present invention, in the ninth aspect of the invention, the optical scanning apparatus is characterized in that the scanning lens having the light-beam passage for allowing a light beam traveling toward the synchronization detecting means to pass through is formed by injection molding, the light-beam passage is formed at an end in the longitudinal direction of the scanning lens, and an injection port used during the injection molding is formed at the other end.
According to a thirteenth aspect of the present invention, in the ninth aspect of the invention, the optical scanning apparatus is characterized in that the scanning lens is a molded lens made of a plastics material.
According to a fourteenth aspect of the present invention, in the tenth aspect of the invention, the optical scanning apparatus is characterized in that the positioning reference is arranged in a symmetrical form in a sub-scanning direction.
According to a fifteenth aspect of the present invention, there is provided an image forming apparatus, characterized by including: an optical scanning apparatus according to any one of the first to fourteenth aspects of the invention; a photosensitive member disposed on a surface to be scanned; a developing device for developing an electrostatic latent image as a toner image, the electrostatic latent image being formed on the photosensitive member with a light beam scanned by the optical scanning apparatus; a transfer device for transferring the toner image developed by the developing device onto a material onto which the toner image is to be transferred; and a fixing device for fixing the toner image transferred on the material onto the material.
According to a sixteenth aspect of the present invention, there is provided an image forming apparatus, characterized by including: an optical scanning apparatus according to any one of the first to fourteenth aspects of the invention; and a printer controller for converting code data input from an external device into an image signal and inputting the image signal to the optical scanning apparatus.
The positioning references are provided at both ends of the scanning optical element to control the position and rotation of the scanning optical element. However, when the spacing between two positioning references for the control with respect to one direction is reduced, the rotation cannot be controlled with high precision.
For example, when an inclined part is present at an end of the scanning optical element, it is difficult to locate a positioning reference member in the inclined part. The positioning reference should therefore be located in a place closer to the optical axis relative to the inclined part. This results in a narrower spacing between the two positioning references for the control with respect to one direction and increased rotational deviation of the scanning/imaging element. Consequently, the aberration on the surface to be scanned is aggravated to cause a problem.
In this case, two rotational deviations are involved.
One is a deviation caused by rotation about an axis parallel to the sub-scanning direction. This rotational deviation is determined by the spacing in the main scanning direction between the two positioning references arranged in the main scanning direction for controlling the position in the optical axis direction and the difference between the position errors in the optical axis direction of the respective positioning references.
The other is a deviation caused by rotation about an axis parallel to the optical axis. This rotational deviation is determined by the spacing in the main scanning direction between the two positioning references arranged in the main scanning direction for controlling the position in the sub-scanning direction and the difference between the position errors in the sub-scanning direction of the respective positioning references.
In addition, the scanning/imaging element has a reduced thickness and thus has a lower stiffness in the inclined part, which is a problem.
The above-mentioned problem is solved by providing the scanning optical element with the light-beam passage for allowing a light beam traveling toward the synchronization detecting means to pass through.
The light-beam passage may be formed, for example, in a notch shape and an opening shape. The positioning reference can be provided within an area where the light-beam passage is present or outside the area, so that it is possible to provide a wide spacing in the main scanning direction between the two positioning references arranged in the main scanning direction for controlling the position in the optical axis direction. Thus, the rotational deviation of the scanning/imaging element about the axis parallel to the sub-scanning direction can be controlled with high precision. Similarly, a wide spacing in the main scanning direction between the positioning references for controlling the position in the sub-scanning direction can be provided. Thus, the rotational deviation of the scanning/imaging element about the axis parallel to the optical axis direction can be controlled with high precision.
The scanning optical element has a reduced thickness only in a portion where the light-beam passage is provided, and a thick portion is present on the outer side of the light-beam passage. As a result, the stiffness deterioration amount can be suppressed to be very small. Particularly, the portions of the scanning optical element where the positioning references are provided are required to have sufficient stiffness for enduring the force applied to the scanning optical element by a spring or the like. In this respect, the light-beam passage has a suitable form.